Capacitance gage for measuring small distances

ABSTRACT

This invention is a capacitance gage for measuring small distances wherein the frequency of an R-C oscillator is varied in proportion to changes in the capacitance responsive to changes in separation between a plate mounted with the gage and a grounded object whose distance is being measured. The gage plate is connected to the input of a Schmitt trigger circuit by means of a field effect transistor amplifier. The output of the Schmitt trigger circuit controls an electronic switch which alternately charges and discharges the probe capacitance in response to the output of the Schmitt trigger circuit between upper and lower voltage triggering levels and the charging frequency is taken in digital form at the output of the switch. The variable frequency output is suitable for high-accuracy, non-contact dimensional inspection with digital readout or direct computer processing. The gage further provides improved performance for medium to high-resistance materials and can be compensated to provide a low temperature drift.

United States Patent 1 Henry [54] CAPACITANCE GAGE FOR MEASURING SMALLDISTANCES represented by the United States Atomic Energy Commission [22]Filed: June 3, 1971 [21] Appl. No.: 149,678

[52] US. Cl ..324/61 R, 324/60 CD [51] Int. Cl. ..G01r 27/26 [58] Fieldof Search ..324/60 CD, 60 R, 61 R, 61 S [5 6] References Cited UNITEDSTATES PATENTS 3,584,297 6/1971 Koski ..324/60R 3,452,273 6/1969 Foster..324/61S Primary ExaminerAlfred E. Smith Attorney-Roland A. Anderson11] 3,716,782 51 Feb. 13, 1973 [57] ABSTRACT This invention is acapacitance gage for measuring small distances wherein the frequency ofan R-C oscillator is varied in proportion to changes in the capacitanceresponsive to changes in separation between a plate mounted with thegage and a grounded object whose distance is being measured. The gageplate is connected to the input of a Schmitt trigger circuit by means ofa field effect transistor amplifier. The output of the Schmitt triggercircuit controls an electronic switch which alternately charges anddischarges the probe capacitance in response to the output of theSchmitt trigger circuit between upper and lower voltage triggeringlevels and the charging frequency is taken in digital form at the outputof the switch. The variable frequency output is suitable forhigh-accuracy, non-contact dimensional inspection with digital readoutor direct computer processing. The gage further provides improvedperformance for medium to high-resistance materials and can-becompensated to provide a low temperature drift.

9 Claims, 2 Drawing Figures 1 w I n memu. RCORDER FREQUENCY METER sv DPATENIE FEB 1 31973 muhui ozuzommm FIIIIIIIIM P H l I I I l I l I I |1|INVENTOR. John J. Henry BY CAPACITANCE GAGE FOR MEASURING SMALLDISTANCES BACKGROUND OF THE INVENTION The present invention was madeduring the course of, or under, a contract with the United States AtomicEnergy Commission.

This invention relates generally to apparatus for noncontact measurementof small distances and more specifically to an improved capacitance gagefor measuring small distances which provides direct digital readout.

Capacitance gages have been used for non-contact dimensional inspectionfor a number of years. Several well known analog circuits such as thecapacitance bridge, the resonant slope detector and the guardedimpedance meter have been used in these measuring circuits. The circuitis used to measure an extremely small capacitance between the probe tipand the item to be inspected, with the output signal displayed on ananalog meter or recorder. The basic means employed to measure these verylow capacitances uses one of the following techniques: (1) the amplitudeof an AC voltage or current is varied by the capacitor, converted to aDC signal and displayed; or (2) an oscillator frequency is varied by thecapacitor, this frequency is then converted to a DC signal anddisplayed. All of these systems are basically analog instruments and,even in the existing oscillator types, it is necessary to convert froman analog to a digital signal for computer processing. Most of theexisting oscillator types are active only for a limited range in theoperating region, and they show gross non-conformity near contact withthe part. This problem makes accurate absolute dimension measurementextremely difficult due to the difficulty in obtaining an absolute zero.With the advent of the digital computer for data processing, it isadvantageous to use a digital readout, such as a variable frequency,directly into the computer in order to avoid the errors introduced inconverting first to a DC signal and then from the analog to a digitalsignal. As previously mentioned, however, existing variable-frequencycapacitance gages lack the accuracy, stability, and conformity necessaryto advantageously use digital data processing.

Other variable frequency measuring devices such as those based on theWein bridge principle, are known in the art in which the gatecapacitance is used as the variable element of a Wein bridge R-Coscillator. These circuits are complicated by the requirement of zerophase shift and nearly equal amplitudes at the bridge midpoints. Thezero phase shift requirement demands that the phase angle through thetwo halves of the tuned side of the bridge must be equal and opposite,thus limiting the measurement range of this type gage. The probe platesize in gages of this type limits the range of sensitivity, as thedistance from the part increases substantially in proportion to theprobe plate area the gage becomes relatively insensitive.

In addition both the LR and Wein bridge-type gages exhibit a reciprocalsquare root theoretical calibration curve which increases the curvatureof the equation causing more stringent requirements on frequencystability and conformity at the greater separation distances; and arenot extrapolatable to zero separation distance making absolute distancecalculation difficult.

SUMMARY OF THE INVENTION In view of the deficiencies in capacitance typedistance gages of the prior art, it is an object of this invention toprovide an improved capacitance gage with direct digital output.

Another object of this invention is to provide a capacitance gage of thevariable frequency type with improved accuracy for non-contactdimensional in- 0 spection with digital readout.

Yet another object of this invention is to provide a capacitance gagewhich provides improved performance for gaging medium to high resistancematerials.

Still another object of this invention is to provide an improvedcapacitance gage which can be readily compensated to provide a lowtemperature drift.

These and other objects of the present invention are generally achievedin a distance gage for measuring the distance of an electricallygrounded object from a gage plate of the gage, wherein: .a fixedresistance circuit means is connected to the gage for providing acharging current to the capacitance formed by the spacing between theobject and the plate; a buffer amplifier having a high input impedanceand a low output impedance connects the gage plate to the input of atrigger circuit; a switching means is connected to the output of thetrigger circuit which switches said charging current off" and on inresponse to the activation of the trigger circuit each time thecapacitance is charged to a predetermined level; and digital recordingmeans is connected to the charging circuit for recording the chargingfrequency of the capacitance, whereby a change in capacitance inresponse to a change in the spacing between the gage plate and theobject is recorded as a proportional change in charging frequency.

Other objects and many of the attendant advantages of the presentinvention will be obvious from the following detailed description takenin conjunction with the accompanying drawings wherein FIGS. 1A and 1Bshow a detailed circuit diagram of a capacitance gage according to thepresent invention.

DETAILED DESCRIPTION Referring to the drawing, an electrically groundedworkpiece 5, the surface of which is to be measured without touching theworkpiece, is arranged close to a sensing means in the form of acapacitance probe 7. The probe 7 is preferably in the form of a circulardisk plate which forms one element of a capacitor 9 with the workpieceas the other element.

A typical probe design includes a 56-inch diameter metal tube (notshown) which houses a printed circuit board containing the electronicsshown enclosed by dotted lines 11. The probe plate 7 may be attached tothe circuit board by means of a threaded shaft 13, extending from oneend of the tube so that a replaceable gage plate of any desired size maybe attached to the threaded shaft. The size and shape of the gage plate7 affects the gage sensitivity and gaging range. Gaging plates from 3/ l6-inch in diameter to 1 inch square have been used with excellentresults, but the more common sizes are its, 1% and inch in diameter.

The circuit board with attached cable extending from the opposite end ofthe tube may be potted into the tube in a conventional manner or mountedbetween Teflon end seals depending upon the application. The tube may beconnected to system ground in order to insure proper shielding.

The shaft 13 is connected to the gate of a field effect transistor (PET)amplifier 15 which has the drain connected to a regulated positivevoltage source, typically +25 volts DC, and the source connected toground through a variable source resistor 17. The output of amplifier15, taken at the source, is connected to the input of a Schmitt triggercircuit 19. The Schmitt trigger circuit is of a design wherein the inputis connected to the base electrode of a first transistor 21 which hasthe collector connected to the volt DC source through a resistor 23 andthe emitter connected to a regulated negative DC voltage source througha resistor 25. The output of the first transistor 21, taken at thecollector, is connected to the base electrode of a second transistor 27through a voltage divider network of series resistors 29 and 31connected between the collector of transistor 21 and ground potential.The emitters of transistors 21 and 27 are connected in common and thecollector of transistor 27 is connected to the volt DC source throughresistor 33. The output of the Schmitt trigger circuit, taken at thecollector of transistor 27 is connected to the input of an electronicswitch 35. The switch 35 consists of a single transistor 34 having itsbase electrode connected to the output of trigger circuit 19 and itsemitter connected to the volt DC source through a resistor 37. Theoutput of switch 35, taken at the collector of transistor 34, isconnected to a point 39 in the circuit.

The charging voltage at point 39 for capacitor 9 is supplied through theswitching transistor 34 when it is conducting and is applied to plate 7by means of a charging circuit consisting of a resistor 41 and atemperature compensating thermistor 43 serially con nected between point39 and shaft 13. The charging circuit further consists of a voltagecalibration circuit 45 connected to point 39 through a voltage clampingdiode 47 with the anode of diode 47 connected to point 39. Thecalibration circuit (FIG. 18) consists of a transistor 49 having itsemitter connected to the cathode of diode 47 and its collector connectedto ground. The base of transistor 49 is connected to the wiper ofpotentiometer 51 which supplies a voltage to the base of the calibratecircuit transistor 49 which then acts through its emitter circuit anddiode 47 as a clamp on any positive going voltage at the output of theelectronic switch (point 39). The value of the clamping voltage isestablished by connecting one end of potentiometer 51 to ground and theother end to a source of positive voltage (typically +15 volts DC)through a series resistor 53.

The purpose of the calibration circuit 45 is to provide adjustment forthe DC charging voltage impressed on point 39 when transistor 34 isconducting. The voltage limit at which diode 47 clamps is determined bythe setting of potentiometer 51, typically in the range of between 6 and15 volts positive.

The output signal from the circuit is also taken at point 39. A pair ofseries resistors 55 and 57 may be connected between point 39 and ground,thereby providing a voltage divider for a readout device, such as adigital frequency meter 59 or computer input connected to the commonconnection between resistors 55 and 57. The output of meter 59 may berecorded if desired on a conventional digital recorder (or printer) 61connected to meter 59.

in operation, the regulated voltage applied to the col lector oftransistor 34 is limited at point 39 to a moderate positive voltage ofabout 16 volts depending upon the setting of potentiometer 51. Thislimited voltage or calibrate" voltage, is applied to the gage plate 7through thermistor 43 and resistor 41 so as to charge the variablegaging capacitor 9 of the probe. The charging voltage on capacitor 9 issensed at the gate of FET 15 which acts as a buffer amplifier. Since FET15 operates with about -H .2 volt bias from source to gate, FET 15 willbegin to conduct when the circuit is first energized and vary thevoltage dropped across resistor 17 which is the input to the Schmitttrigger circuit 19. The FET buffer amplifier 15 is used to couple theR-C circuit of the gage to the required low input impedance of theSchmitt trigger circuit 19. As a result, when the voltage on capacitor 9is about +4.8 volts, the conduction through FET 15 is sufficient toprovide a voltage drop across resistor 17 which exceeds the uppervoltage triggering level of the Schmitt trigger circuit 19. When theSchmitt trigger circuit 19 triggers at the upper level, it turns off theelectronic switch 35 (transistor 34), causing point 39 to go towardground potential. Capacitor 9 then discharges throughresistors 41, 43,55 and 57 until the output of amplifier 15 reaches the lower triggeringvoltage level of Schmitt trigger 19 (about 3.4 volts across capacitor9). The Schmitt trigger 19 then turns the switch 35 back "on", applyingthe calibrate voltage to the input R-C circuit and repeating the cycle.

The alternate application of the positive voltage and ground potentialis repeated at a definite period determined by the value of thecalibrate voltage (point 39), the value of the series resistor 41, thevoltage levels of the upper and lower Schmitt triggeririg points, andthe value of the active element-capacitor 9. The output read by meter 59is the frequency or period of the switching between these two stablestates and for the circuit shown is typically in the range from 0-600Khz. The output is a constant amplitude square wave the frequency ofwhich varies in proportion to the distance (d) between the groundedobject 5 and the probe gage plate 7. The requirements for good stableoperation of this circuitry determine the circuit configuration andoperating conditions. The most satisfactory input buffer field effecttransistor found was the commercially available 2N44l6 supplied by UnionCarbide Corporation, Electronics Division, San Diego, California. Allother junction and metal oxide semiconductor FETs examined had too highan input capacitance, or input current leakage, or both.

The Schmitt trigger circuit is of a special design and requireswell-regulated +25 and 15 volts power supplies for stable triggerlevels. The upper level is determined primarily by the 25-volt supplyand the resistors 23, 29 and 31. This path can be made very stable. Thelower level is determined from the upper level by the collector currentof transistor 27 in the on" state. This path is made very stable using alarge resistance in the emitter circuit (resistor 25) and awell-regulated l5 volts power supply.

The temperature characteristics of this probe may be adjusted bychanging or adjusting the value of resistor 17 in the source lead of PET15. Excellent agreement between stray input capacities and temperatureresponse using a standard value resistor was obtained with this probe.Two designs of this probe have been verified, one employing a 4.7Ksource resistor 17 which optimizes the probe for wide temperatureexcursion and a probe using a 6.8K resistor which has a minimumtemperature coefficient around room temperature operation. To offsettemperature changes in the charging circuit impedance a K thermistor 43is used in series with the charging circuit resistor 41.

The circuit configuration is such that zeroing is not required as inmost non-contact measuring devices, since the output will be zero hertzwhen contact is made between the part 5 and the probe plate 7,regardless of other calibration factors. Calibration is accomplished byadjusting the output to a desired frequency when the probe plate andpart are positioned a precise distance apart. The calibrating frequencyis varied by changing the value of the charging voltage at point 39 byadjusting potentiometer 51.

Stability tests run on this probe yield an accuracy of $0.1 mil at 40mil separation of the input probe plate 7 from the grounded object 5 andthe effect of temperature varied between and 55C was $0.2 mil. Thefrequency stability of the oscillator of this probe built with allcomponents designed for low drift is very good, being in the order of 1part in 10.

Due to the high input impedance, measurements to moderately resistiveparts may be made with negligible calibration changes from a referencecalibration on a good conductor object.

Accordingly, it will be seen that a highly accurate and simplenon-contact capacitance gage has been provided which may be used fordirect digital computer processing not only for dimensional inspectionbut for dielectric constant or dielectric homogeneity measurements ofsuch materials as plastics, fiberglass resin compositions, and paintfilms, when a separate thickness measurement is made by other gagingmethods and a conductive backing is provided. in this case, thedielectric constant is the ratio of the actual dielectric thickness tothe (air calibrated) indicated thickness from the capacitance probe.

What is claimed is:

1. A distance gage for non-contact measurement of A the distance of anelectrically grounded object from a probe plate mounted with said gage,comprising:

circuit means connected to said plate for providing a charging currentto the capacitance formed by the spacing between said object and saidplate;

a buffer amplifier having a high impedance input and a low impedanceoutput, said input of said amplifier being connected to said plate forsensing the charging voltage developed across said capacitance;

a trigger circuit connected to the output of said amplifier and havingpredetermined upper and lower voltage triggering levels, said triggercircuit having an output whose signal level is switched in triggercircuit and charging cir cuit means for SW] chlng said charging currentof and on" responsive to the activation of said trigger circuit;

and

means connected to said charging circuit means for recording thecharging frequency of said capacitance, whereby a change in saidcapacitance due to a change in the spacing between said object and saidplate is recorded as a proportional change in charging frequency.

2. A distance gage as set forth in claim 1 wherein said buffer amplifierincludes a field effect transistor having gate, source and drain, saidgate connected to said gage plate and said drain connected to a highlyregulated source of positive voltage; and a source resistor connectedbetween said source and ground potential across which the output of saidamplifier is taken.

3. A distance gage as set forth in claim 2 wherein said trigger circuitis a Schmitt trigger circuit having highly regulated upper and lowertriggering levels.

4. A distance gage as set forth in claim 3 wherein said switching meansincludes a switching transistor having base, emitter and collectorelectrodes, said base electrode connected to the output of said Schmitttrigger circuit, said emitter electrode resistively coupled to saidregulated power source and said collector electrode connected in serieswith said charging circuit means.

5. A distance gage as set forth in claim 4 wherein said charging circuitmeans includes a charging resistor connected between said emitter ofsaid switching transistor and said gage plate.

6. A distance gage as set forth in claim 5 wherein said charging circuitmeans further includes a calibration circuit means connected between thecollector of said switching transistor and ground potential forselectively controlling the charging voltage to said gage plate, therebyproviding a predetermined operating frequency at a given spacing of saidgage plate from the surface of said object.

7. A distance gage as set forth in claim 6 wherein said source resistoris a variable resistor, whereby the bias voltage on said field effecttransistor may be varied so as to provide temperature compensation ofthe gage operating'frequency.

8. A distance gage as set forth in claim 7 wherein said charging circuitfurther includes a thermistor connected in series with said chargingresistor so as to stabilize the charging resistance for variationtemperature during operation.

9. A distance gage as set forth in claim 8 wherein said recording meansincludes a pair of resistors serially connected between said emitter ofsaid switching transistor and ground potential, thereby providing adischarge path for said capacitance when said switching transistor is inthe non-conducting state and a digital frequency recording meansconnected to the common connecting point of said serially connectedresistors. s

i i i i i

1. A distance gage for non-contact measurement of the distance of anelectrically grounded object from a probe plate mounted with said gage,comprising: circuit means connected to said plate for providing acharging current to the capacitance formed by the spacing between saidobject and said plate; a buffer amplifier having a high impedance inputand a low impedance output, said input of said amplifier being connectedto said plate for sensing the charging voltage developed across saidcapacitance; a trigger circuit connected to the output of said amplifierand having predetermined upper and lower voltage triggering levels, saidtrigger circuit having an output whose signal level is switched inresponse to said upper and lower triggering levels; switching meansconnected to the output of said trigger circuit and charging circuitmeans for switching said charging current ''''off'''' and ''''on''''responsive to the activation of said trigger circuit; and meansconnected to said charging circuit means for recording the chargingfrequency of said capacitance, whereby a change in said capacitance dueto a change in the spacing between said object and said plate isrecorded as a proportional change in charging frequency.
 1. A distancegage for non-contact measurement of the distance of an electricallygrounded object from a probe plate mounted with said gage, comprising:circuit means connected to said plate for providing a charging currentto the capacitance formed by the spacing between said object and saidplate; a buffer amplifier having a high impedance input and a lowimpedance output, said input of said amplifier being connected to saidplate for sensing the charging voltage developed across saidcapacitance; a trigger circuit connected to the output of said amplifierand having predetermined upper and lower voltage triggering levels, saidtrigger circuit having an output whose signal level is switched inresponse to said upper and lower triggering levels; switching meansconnected to the output of said trigger circuit and charging circuitmeans for switching said charging current ''''off'''' and ''''on''''responsive to the activation of said trigger circuit; and meansconnected to said charging circuit means for recording the chargingfrequency of said capacitance, whereby a change in said capacitance dueto a change in the spacing between said object and said plate isrecorded as a proportional change in charging frequency.
 2. A distancegage as set forth in claim 1 wherein said buffer amplifier includes afield effect transistor having gate, source and drain, said gateconnected to said gage plate and said drain connected to a highlyregulated source of positive voltage; and a source resistor connectedbetween said source and ground potential across which the output of saidamplifier is taken.
 3. A distance gage as set forth in claim 2 whereinsaid trigger circuit is a Schmitt trigger circuit having highlyregulated upper and lower triggering levels.
 4. A distance gage as setforth in claim 3 wherein said switching means includes a switchingtransistor having base, emitter and collector electrodes, said baseelectrode connected to the output of said Schmitt trigger circuit, saidemitter electrode resistively coupled to said regulated power source andsaid collector electrode connected in series with said charging circuitmeans.
 5. A distance gage as set forth in claim 4 wherein said chargingcircuit means includes a charging resistor connected between saidemitter of said switching transistor and said gage plate.
 6. A distancegage as set forth in claim 5 wherein said charging circuit means furtherincludes a calibration circuit means connected between the collector ofsaid switching transistor and groUnd potential for selectivelycontrolling the charging voltage to said gage plate, thereby providing apredetermined operating frequency at a given spacing of said gage platefrom the surface of said object.
 7. A distance gage as set forth inclaim 6 wherein said source resistor is a variable resistor, whereby thebias voltage on said field effect transistor may be varied so as toprovide temperature compensation of the gage operating frequency.
 8. Adistance gage as set forth in claim 7 wherein said charging circuitfurther includes a thermistor connected in series with said chargingresistor so as to stabilize the charging resistance for variationtemperature during operation.